The use of membranes to separate aromatics from saturates has long been pursued by the scientific and industrial community and is the subject of numerous patents.
U.S. Pat. No. 3,370,102 describes a general process for separating a feed into a permeate stream and a retentate stream and utilizes a sweep liquid to remove the permeate from the face of the membrane to thereby maintain the concentration gradient drive force. The process can be used to separate a wide variety of mixtures including various petroleum fractions, naphthas, oils, hydrocarbon mixtures. Expressly recited is the separation of aromatics from kerosene.
U.S. Pat. No. 2,958,656 teaches the separation of hydrocarbons by type, i.e., aromatic, unsaturated, and saturated, by permeating a portion of the mixture through a non-porous cellulose ether membrane and removing permeate from the permeate side of the membrane using a sweep gas or liquid. Feeds include hydrocarbon mixtures, e.g., naphtha (including virgin naphtha, naphtha from thermal or catalytic cracking, etc.).
U.S. Pat. No. 2,930,754 teaches a method for separating hydrocarbons, e.g., aromatic and/or olefinic, from gasoline-boiling range mixtures by the selective permeation of the aromatic through certain nonporous cellulose ester membranes. The permeated hydrocarbons are continuously removed from the permeate zone using a sweep gas or liquid.
U.S. Pat. No. 4,115,465 teaches the use of polyurethane membranes to selectively separate aromatics from saturates via pervaporation. U.S. Pat. Nos. 5,028,685 and 5,093,003 disclose halogenated polyurethanes and membranes therefrom for separating aromatics from saturates.
U.S. Pat. Nos. 4,944,880 and 4,990,275 describe polyimide/aliphatic polyester copolymers and membranes therefrom for the separation of aromatics from saturates. U.S. Pat. Nos. 4,946,594 and 4,997,906 describe crosslinked copolymers of aliphatic polyester diols and dianhydrides and membranes therefrom for the separation of aromatics from saturates.
U.S. Pat. No. 4,976,868 discloses the use of polyester membranes (e.g., polyethylene terephthalate, polybutylene terephthalate, and polyethylene terephthalate/cyclohexane-dimethanol terephthalate) for aromatics/saturates separation. U.S. Pat. Nos. 5,012,036, 5,012,035 and 5,019,666 teach the use of polyarylate, polyphthalatecarbonate, and nonporous polycarbonate membranes, respectively, to separate aromatics from saturates, U.S. Pat. No. 5,055,631 discloses sulfonated polysulfone membranes for the separation of aromatics from saturates. U.S. Pat. No. 5,128,439 describes saturated polyesters and crosslinked membranes therefrom for aromatics/saturates separation.
U.S. Pat. No. 3,966,834 disclose the separation of dienes from organic mixtures using uncrosslinked membranes.
In 1973, F. P. McCandless (Ind. Eng. Chem. Process Des. Develop., 12 (3), 354 (1973)) published a paper on the separation of aromatics from naphthenes by permeation through modified polyvinylidenefluoride membranes. The modifying agent was 3-methyl-sulfolane used as a plasticizer. The maximum temperature used by McCandless was 100.degree. C. In practice, the membranes described by McCandless cannot have suitable stability; the plasticizer would be slowly leached out and contaminate the permeate, and the membrane performance would change with time.
In a subsequent paper, McCandless et al (Ind. Eng. Chem. Process Des. Develop., 13 (3), 310 (1974)) investigated the separation of benzene from cyclohexane using a solvent-modified polyvinylidenefluoride film via adding the solvent in the hydrocarbon feed. The solvents were dimethylformamide (DMF) or dimethylsulfoxide. Such a solvent-modified membrane process cannot be practical in industrial use as it would require the separation of the solvent from the permeate and retentate.
The present invention describes the use of crosslinked fluorinated polyolefin membranes for the separation of aromatics from saturates. As will result from the examples, the crosslinked membranes covered by this invention can be used at temperatures well above those used by McCandless. The word "aromatics" includes, in addition to aromatic hydrocarbons, also heteroatom cyclic compounds containing sulfur and nitrogen.
Compared to distillation, membrane permeation can lead to considerable energy savings. A membrane can separate a mixture of aromatics and saturates, e.g., a heavy catalytic naphtha, into a high-octane, mainly aromatic permeate and a high-cetane, mainly saturated retentate. Both permeate and retentate are more valuable than the starting heavy catalytic naphtha.